Outline

The p53 tumor suppressor pathway is inactivated in most human tumors, either directly by gene mutations or by defects of target genes as well as by expression of regulatory proteins, such as HDM2 and HDMX. HDM2 is the key negative regulator of p53. However, although negative regulatory functions of HDMX towards p53 have been shown in vivo, only a few data exist so far, demonstrating the clinical importance of HDMX and its splice variants (e.g. HDMX-S) and its suitability as a therapeutic target. The aim of our study was therefore to determine the HDMX amplification and expression status in tumors and the effect of a downregulation of HDMX by siRNA in cell lines with different p53 status. Here we show that the HDMX-S transcript is predominantly expressed in 14% of soft tissue sarcomas which was associated with a significantly decreased survival time. Specifically, we observed an up to 16-fold excess of the HDMX-S transcript in comparison to FL-HDMX. The risk of a tumor-related death was 17-fold higher for those soft tissue sarcoma patients. Furthermore, the HDMX gene was amplified in 17% of soft tissue sarcomas. The HDMX gene amplification appeared to be an independent prognostic marker for STS. However, an amplified HDMX gene was not associated with increased HDMX mRNA expression. Furthermore, we have investigated the effects of the siRNA mediated knock-down of HDMX and HDMX-S in several ovarian cancer cell lines with wild-type and mutant p53. We could specifically decrease the level of full-length HDMX in a number of ovarian cancer cell lines up to 90%. Although we detected only a slight decrease of the cell viability in a short-term assay in OAW-42 (wild-type 53) but not in Skov-3 (mutant p53) after a HDMX mRNA knock-down, we observed dramatic morphological changes of the nuclei in both OAW-42 and Skov-3. Currently we are investigating the long-term effects of a HDMX knock-down in other cell lines overexpressing HDMX. Furthermore, we were interested in whether a knock-down of HDMX could sensitize ovarian cancer cells against a treatment with cisplatin and paclitaxel. In our study, we achieved a 1.2-fold sensitization of OAW-42 cells against a paclitaxel treatment following a knock-down of HDMX and HDMX-S. In summary, our data suggest that HDMX plays a role in tumorigenesis. Furthermore, HDMX could be a potential therapeutic target and its knock-down may help to overcome chemoresistance in certain tumor types.